Drilling devices wherein a drill bit is rotated by a downhole motor, e.g. A positive displacement fluid motor, are well known. A positive displacement type motor includes a housing, a stator having a helically contoured inner surface secured within the housing and a rotor having a helically contoured exterior surface disposed within the stator. As drilling fluid or "mud" is pumped through the stator, the rotor is rotated within the stator and also orbits around the internal surface of the stator in a direction opposite the direction of rotation. The rotor is connected to a rotatable drive shaft through a flexible coupling to compensate for the eccentric movement of the rotor.
The application of flexible couplings to positive displacement motors for downhole drilling is very challenging due to an extremely corrosive and erosive operating environment and constraints on length and diameter in view of the very heavy loads that must be transmitted. Conventional flexible coupling designs use moving parts, e.g. universal joints of the type described in U.S. Pat. No. 3,260,069 (Nielson et al), to compensate for eccentric movement of the rotor and for shaft misalignment. Jointed flexible couplings provide a short service life in downhole applications, due to severe wear problems associated with the moving parts of such couplings.
Moineau motors in which a flexible connection between a drive shaft and rotor are provided by a flexible shaft, rather than jointed rigid members are described in U.S. Pat. No. 2,028,407 (Moineau) and U.S. Pat. No. 4,679,638 (Eppink). Moineau provides no guidance as to how to secure a flexible shaft to a rotor and to a drive shaft in a manner which will withstand the severe thrust, torsion and bending loads encountered in downhole motor application. Eppink describes one approach to interconnecting the rotor, flexible shaft and drive shaft in the form of a tapered threaded fittings and a pin (element 61 of Eppink). Threaded connections and pinned connections introduce stress concentrations into the flexible coupling which can give rise to fatigue failures and thereby compromise the service life of the coupling. Components of the shaft assembly described by Eppink are not interchangeable and the entire assembly must be replaced if one of the components of the assembly fails.